Glaucoma is one of the leading causes of blindness, resulting from the loss of a particular type of retinal cells, more specifically, retinal ganglion cells (RGC). Axons of the ganglion cells project out of the eye to form the optic nerve. With the loss of ganglion cells, the optic nerve which connects the retina to the brain is gradually destroyed, and results in blindness if the disease is not treated early enough.
Although typical signs of glaucoma include a scotoma as well as a "cupping" of the optical disc, by the time such signs are detected, treatment is unlikely to be successful. While glaucoma is typically associated with an elevation in a patient's intraocular tension, testing for such an elevation is generally unreliable. This is so, since an elevated intraocular tension typically occurs transiently in the morning and evening, or may not even be exhibited by some patients.
As such, an evaluation of the patient's visual field has long been the method used for the clinical diagnosis of glaucoma, as well as other pathologies. For example, in so-called "white-on-white" perimetry, a white test object of varying contrast is displayed against a white background at different points in the patient's visual field. The characteristic locations where the test object is undetected for a particular contrast allow clinicians not only to diagnose, but also to determine the severity of the glaucoma. A drawback, however, to this approach is its lack of sensitivity, typically detecting glaucoma only after 30-50% of retinal ganglion cells have been lost or destroyed.
Efforts to improve the sensitivity of the above latter approach have focused on the physiology of retinal ganglion cells. For example, in "short-wave" automated perimetry (SWAP), a blue rather than a white test object is displayed within the patient's visual field, and against a yellow background. The characteristic locations where the test object is undetected for a particular contrast are used to effectively screen patients for glaucoma damage.
Recently, another approach using the phenomenon of so-called "frequency doubling" has also been used for the early detection of glaucoma. See, for example, U.S. Pat. No. 5,065,767, which is incorporated herein by reference. As shown in FIG. 1, in this latter approach, a sinusoidal grating pattern 10 consisting of light and dark bars or striations 20, 30, respectively, is modulated at a temporal frequency between 10 and 50 Hz. That is, the bars are contrast modulated in a sinusoidal fashion from white through gray to black at about 10 to 50 times a sec. At such frequencies, typically about 20 Hz, the grating pattern is perceived by patients to have double the spatial frequency. For a discussion on this phenomenon, see, for example, D. H. Kelly, "Frequency Doubling In Visual Response," J. Opt. Soc. Am., 56:1628-33 (1966); and D. H. Kelly, "Nonlinear Visual Responses To Flickering Sinusoidal Gratings," J. Opt. Soc. Am. 1051-55 (1981).
Patients suffering from glaucoma typically require twice the contrast level between the white and black bars before observing the above frequency doubling phenomenon. This phenomenon is now understood to be a non-linear visual response of the eye. Importantly, this difference in visual response between patients with normal vision and those suffering from glaucoma is used to detect the disease at an earlier stage.
Although the above latter approaches perform satisfactorily, it would still be desirable to have a glaucoma test which has greater sensitivity, so as to detect glaucoma at its earliest possible stage.